The invention relates to a method for manufacturing heat-stable structured layers, as well as to the use of these structured layers.
Methods for the manufacture of structured layers based on heat-stable polymers are known, for example from the German patent document 2 308 830 as well as the European patent documents 0 019 123, and 0 026 820. In these methods soluble, photoreactive intermediary stages of highly heatstable polymers are used for photolithographic structuring and the structures thereby produced are cyclized in a subsequent tempering step. For the complete cyclization and removal of the cleavage products temperatures of up to 400.degree. C. are required. This requires thermally highly stressable substrates.
In the fields of circuits and conductors, for example in circuit board technology, substrates based on epoxides are used, which can be thermally stressed up to maximally approximately 150.degree. C./hour and which need to be able to tolerate temperatures of approximately 280.degree. C. only for seconds, for instance during soldering processes. The solder resist lacquers used here to partially cover conducting paths must meet similar thermal demands, i.e. here, for covering the sites of the circuit surface, which must not make contact with the soldering metal, polymers of medium thermal stability are required. The dry resists used at present for this purpose, including respectively silk screening lacquers, based upon epoxides and upon acrylates, meet the requirements of the function of a solder resist mask. However, they only partially fulfill the increased demands with respect to pattern outline accuracy measuring accuracy in the microelectronics field with structures of less than 100 .mu.m, as well as the requisite cycling strength. To this end, photolithographic lacquer systems are required.
A photostructurable lacquer system based on epoxides with chalcon groups built into the polymer chains, i.e. groups having the structure --C.sub.6 H.sub.4 --CH.dbd.CH--CO--C.sub.6 H.sub.4 --, is already available, which would ensure sufficient pattern outline accuracy (see "Chimia", Vol.38 (1984), pages 13 to 20). With respect to the photostructurability, however, relatively long exposure times and, in particular long developing times are involved. Beyond that, use of these known lacquer systems to establish additional permanent protection of the circuit surface against damaging gases, often requires elaborate multiple coating processes. Furthermore, due to the lengthy after-curing times sometimes lasting several hours, the overall process is long and cost-intensive.
An object of the invention, therefore, is to develop a method for making heat-stable structured layers which permit producing precisely dimensioned, high-quality structured layers, in particular on circuit surfaces, in a single coating process. Another object is to develop resistance against the extensive thermal and mechanical stresses, for example, during immersion soldering processes, and, moreover, protect the circuit surfaces effectively and permanently against moisture and corrosion. In addition, it is an object to develop an overall process that is shortened by shorter exposure, developing and tempering times and thus becomes more costeffective. Yet other objects include a developing process which yields highly resolved structures, with the additional possibility of permitting working in aqueous/alkaline media.